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Experiment 15

Experiment 15. Chemical Kinetics. Purpose. The purpose of this experiment is to determine the rate of a chemical reaction (potassium permanganate, KMnO 4 , + oxalic acid, H 2 C 2 O 4 ) as the concentrations are varied and to determine the rate law for the reaction. Introduction.

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Experiment 15

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  1. Experiment 15 Chemical Kinetics

  2. Purpose • The purpose of this experiment is to determine the rate of a chemical reaction (potassium permanganate, KMnO4, + oxalic acid, H2C2O4) as the concentrations are varied and to determine the rate law for the reaction.

  3. Introduction • For a reaction a A + b B  c C + d D

  4. Different Rates • Average (this experiment) • Initial (Experiment 17) • Instantaneous

  5. Rate Law • Rate law---a relationship between concentrations and rate. For a reaction aA + bB  products the rate law often takes the form Rate = k[A]x[B]y

  6. Rate = k[A]x[B]y • [A],[B]: molarities of A and B in solution • x, y: orders with respect to A and B, respectively. (These orders might not correspond to coefficients from the balanced equation!) • k: rate constant

  7. What We’re Running • 2 MnO4- + 5 H2C2O4 + 6 H+ 2 Mn2+ + 10 CO2 + 8 H2O • We assume a rate law Rate = k [MnO4-]m[H2C2O4]n • The rate law and rate constant are not affected by concentration.

  8. Strategy • One pair of experiments (assignments 1 and 2): [MnO4-] is constant and [H2C2O4] doubles. • In another pair (1 and 3), [MnO4-] doubles and [H2C2O4] is constant. • For each of these pairs, divide the rate measured in one experiment by that from another.

  9. Comparing assignments 1 and 2: • This can be rewritten as • Since rates and concentrations are known, n (the order with respect to oxalic acid) is available.

  10. Similarly, comparing assignments 1 and 3 gives • And, therefore, From this, m (the order with respect to MnO4-) is available.

  11. Once the orders are known, we can calculate the rate constant from the rate law. • Since rates depend on temperature. we will also look at the effect of temperature on the rate of this reaction.

  12. Safety • Aprons and glasses. • KMnO4 is a strong oxidant (and also stains skin and clothing); oxalic acid is poisonous. • Waste into waste bottles.

  13. Safety 2 • If you use the Bunsen burner for heating water, keep hair, clothing, paper, and other flammable material away. • Shut off burner before mixing high-temperature samples.

  14. Procedure • Work in pairs. • Check out pipettes and bulbs from stockroom. • Needed equipment: medium-sized test tubes; 250- and 400-mL beakers. May also need ring stand, ring, wire gauze, and Bunsen burner.

  15. Quantities of Reactants Used

  16. Mark an X on a piece of white paper. • Get 75 mL oxalic acid and 15 mL KMnO4 solutions; record concentrations. • Two tubes for each assignment; pipet desired volumes of oxalic acid and water into each tube. For use of volumetric pipets, review Expt. 11 from CHEM 1031.

  17. Prepare four additional tubes for Assignment #1; also pipet 1.0 mL of KMnO4 solution into each of four small test tubes. Save these for temperature study---last part of experiment. • Start with first oxalic acid-water tube for Assignment 1. Place the paper behind the test tube.

  18. Pipet KMnO4 solution into tube; begin timing when half the solution has been added. • When you can see the X through the test tube, record elapsed time on your data sheet. • Repeat the run you have just completed; then do duplicate runs for the other two assignments.

  19. Effect of temperature: Place two oxalic acid tubes and two KMnO4 tubes into a beaker containing warm water (10oC above room temperature). (If tap water is not sufficiently warm, use Bunsen burner.) • After tubes have been in warm water for 5 minutes or so, add KMnO4 to an oxalic acid-water mix; record elapsed time for X to become visible. Repeat.

  20. Make cool water bath (ice in water) to get 10oC below room temperature. • Cool other two oxalic acid-water mixes and KMnO4 samples. Again mix, record elapsed time as before. Repeat.

  21. Calculations • Concentrations of oxalic acid and KMnO4 from dilution formula: • Example: 5.00 mL of 0.755 M oxalic acid diluted to 12.00 mL gives 0.315 M.

  22. For each assignment, average the times for the two runs. • D[MnO4-] = [MnO4-]f – [MnO4-]i = -[MnO4-]i

  23. From measured rates, determine orders: • Round the orders to the nearest integers.

  24. Go back to the rate law: Rate = k [MnO4-]m[H2C2O4]n • You now know rates, concentrations, and orders. Calculate k for each assignment and average.

  25. The effect of temperature • Rate is proportional to DConcentration/Dtime • ---if the time decreases by a factor of 3 (say), the rate correspondingly increases by a factor of 3.

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